Elegant way to adjust this angle (which has another object which must stay parallel to it)
see my response to Nascent space above,
the missing link was to then change 'transform orientation' to normal, which allows me to scale the face along its face.
BUT not as elegant as hoped, because i ended up having to carefully line up the bottom vertices with the face of that bottom plate. error there doesn't bother me as much, but its not perfect, so still interested in ideas